Blender 2.8 : enter 'select box' mode while in transform mode
You need to configure key-map.
Find tool Select box, copy operator name view3d.select_box and paste it into "Tweak" sections of Grab, Scale ets (where you want to use it)



If tool haven't "Tweak" section, you can create it by pressing add new and copy settings from other one



In this gif, I don't use B or any outer keyboard buttons